This invention relates to an oxygen concentration-sensing device of the type which generates an output proportional to the concentration of oxygen in a gaseous substance such as exhaust gases emitted from an internal combustion engine.
An air-fuel ratio control system for an internal combustion engine is known e.g. from Japanese Pat. Publication (Kokoku) No. 55-3533, which senses the concentration of oxygen in exhaust gases emitted from the engine by means of an oxygen concentration sensing device, and controls the air-fuel ratio of a mixture supplied to the engine to a desired value in a feedback manner responsive to the output from the oxygen sensor, to thereby purify the exhaust gases and improve the fuel consumption, etc.
The above-mentioned oxygen concentration-sensing device for use in an air-fuel ratio control system includes a type which generates an output proportional to the concentration of oxygen contained in the exhaust gases, i.e. the air-fuel ratio in the exhaust gases. An oxygen concentration-sensing device of this type is disclosed, e.g. in Japanese Provisional Pat. Publication (Kokai) No. 59-192955, which comprises an oxygen-pumping element and a cell element, each being composed of a plate-like member formed of a solid electrolytic material having oxygen ion-conductivity, and a couple of electrodes attached to opposite side surfaces of the plate-like member. A gas-staying chamber is partly defined by one of the electrodes of each of the oxygen-pumping element and the cell element. A gas to be examined is introduced into the gas-staying chamber through a gas-introducing slit. An air chamber into which the atmosphere is introduced is provided adjacent the cell element, with the other of the coupled electrodes of the cell element facing the interior of the air chamber.
According to this oxygen concentration-sensing device, in order to maintain the concentration of oxygen present within the gas-staying chamber at a predetermined value (e.g. 0), a voltage developed across the cell element is compared with a predetermined reference value, and pumping current is caused to flow between the two electrodes of the oxygen-pumping element in response to the result of the comparison. The value of the pumping current is outputted as an output proportional to the oxygen concentration in the gas to be examined.
In the above proportional output-type oxygen concentration-sensing device, as stated above, the concentration of oxygen within the gas-staying chamber is controlled in a feedback manner by varying the pumping current flowing in the oxygen-pumping element in response to the voltage developed across the cell element. There can occur phase rotation or phase delay in the feedback system, depending upon the frequency of variation of the pumping current value. If the loop gain of the feedback system is 1 or more at frequencies where the phase rotation exceeds 180 degrees, there can occur oscillation. Such oscillation can easily occur particularly when the pumping current is in a high frequency range, because the detection gain of an oxygen concentration-sensing element composed of the oxygen-pumping element and the cell element, i.e. the amount of change in the voltage developed across the cell element per unit amount of change in the pumping current is small in the high frequency range, as shown in FIG. 1.